问题
In classes that implement INotifyPropertyChanged I often see this pattern :
public string FirstName
{
get { return _customer.FirstName; }
set
{
if (value == _customer.FirstName)
return;
_customer.FirstName = value;
base.OnPropertyChanged("FirstName");
}
}
Precisely the lines
if (value == _customer.FirstName)
return;
are bothering me. I've often did this but I am not that sure it's needed nor good. After all if a caller assigns the very same value I don't want to reassign the field and, especially, notify my subscribers that the property has changed when, semantically it didn't.
Except saving some CPU/RAM/etc by freeing the UI from updating something that will probably look the same on the screen/whatever_medium what do we obtain?
Could some people force a refresh by reassigning the same value on a property (NOT THAT THIS WOULD BE A GOOD PRACTICE HOWEVER)?
1. Should we do it or shouldn't we?
2. Why?
回答1:
Yes, you should return immediately when the consumer is setting a property value that is equal to the value which is already being persisted.
First of all, there is no reason to waste any time or resources in the setter of the property - the value is already set so no further action is needed. Also you should never call OnPropertyChanged if the value stored in the property's backing field hasn't changed - the method is intended to be raised when the value has changed not when the property's setter has been called.
All that being said, however - if the setter didn't have a call to OnPropertyChanged I wouldn't have bothered to check the value first. In the case of a simple setter that only sets the backing field's value and nothing else it is actually going to be faster to always the set the value rather than checking first then setting the value. Only use this pattern when the property's setter has additional logic that either shouldn't fire or may incur an unnecessary performance penalty.
回答2:
Or you could do this:
set
{
if (value != _customer.FirstName)
{
_customer.FirstName = value;
base.OnPropertyChanged("FirstName");
}
}
No need for multiple return paths.
To further answer your question, I wouldn't force an update to property if it's being overwritten by the same value. There's really no point, because you're probably not going to get any benefit from it. (I could see an instance where you would want to track each time someone tries to update a value.)
回答3:
The only argument against that pattern (where you return if the value hasn't changed) I can think of is the purist's view that every function should have only one exit. Not being a purist, I don't agree. I see nothing wrong with breaking out if the value hasn't changed, avoiding the notification update.
回答4:
The only situation when you shouldn't use it is when you know that you can have dirty data on your class, for example on a ORM layer object that might have outdated values compared to the underlying database because of modification by another user.
If that situation doesn't affect you, then cache away!
Edit
I misunderstood your question, as you are talking about setters, not getters.
Similar points apply. If the set operation is a costly one, and is not supposed to have any kind of side effect (it should't! Side effects on setters are <blink>evil</blink>!!1), then it's a valid optimization.
回答5:
One reason to not return early, I would imagine, is for subscribers that joined the party late. They might not be aware of the object's current state, and will miss out on the setter notification if you return early.
来源：https://stackoverflow.com/questions/2623697/should-a-setter-return-immediately-if-assigned-the-same-value